1) Actual spoilers, which means actually specifying what is under the spoiler box before one clicks on it. Like speculation, spoilers for different series in different forums. Remember to not post actual spoilers!
2) Large pictures to prevent expanding the screen
3) Jokes

Clarification on point 1:
-speculating about events in the future of the series (if you actually KNOW something that happens later in the series, this is not allowed)
-a spoiler for a different series (e.g. talking about something that happened in Gekiranger in the Shinkenger thread)
In both instances a short description of what is in the spoiler tag should be used.

When not to use it:
Spoiler tags or not, never post spoilers of episodes that have not been released by TVN, this includes things from the preview of next weeks episode.

Don't use spoiler tags to talk about the episode that the thread is about. Only the first post should be in spoiler tags, the rest should be easy for people to read. The assumption is that people have seen the episode before reading the thread

Spoiler tags must be used for:
-speculating about events in the future of the series (if you actually KNOW something that happens later in the series, this is not allowed)
-a spoiler for a different series (e.g. talking about something that happened in Gekiranger in the Shinkenger thread)
In both instances a short description of what is in the spoiler tag should be used.

Spoiler tags or not, never post spoilers of episodes that have not been released by TVN, this includes things from the preview of next weeks episode.

Demon beasts called Horrors live in the darkness. This is a story of a Makai Knight who cuts down the Horrors. You better not look away.